1. Field of the Invention
The present invention relates generally to an exhaust emission control system of an internal combustion engine, and more particularly to a catalyst structure for raising a temperature at an early stage.
2. Related Background Art
An exhaust emission control system for purifying NOx from an exhaust gas discharged from an internal combustion engine capable of performing a lean combustion involves the use of, e.g., a NOx absorbent typified by an occlusion reducing type NOx catalyst. The NOx absorbent absorbs NOx when an air-fuel ratio of an inflow exhaust gas is lean (i.e., under an over oxygen atmosphere), and releases absorbed. NOx when an oxygen concentration of the inflow exhaust gas decreases. The occlusion reducing type NOx catalyst classified as one type of this NOx absorbent absorbs NOx when the air-fuel ratio of the inflow exhaust gas is lean (i.e., under the over oxygen atmosphere), and releases absorbed NOx when the oxygen concentration of the inflow exhaust gas decreases, thus reducing it into N2.
If the occlusion reducing type NOx catalyst (which might hereinafter be simply called a catalyst or a NOx catalyst) is disposed in an exhaust passageway of the internal combustion engine capable of performing the lean combustion, the catalyst absorbs NOx contained in the exhaust gas when the exhaust gas having a lean air-fuel ratio flows, and releases NOx, as NO2, absorbed by the catalyst when the exhaust gas having a stoichiometric ratio (theoretical air-fuel ratio) or a rich air-fuel ratio flows. Further, this is reduced into N2 by a reducing component such as HC and CO contained in the exhaust gas, thereby purging the NOx.
By the way, generally a temperature of the catalyst including that type of catalyst must be raised up to a predetermined temperature region in order to purify the exhaust gas.
Accordingly, it is desirable in terms of effectively purifying the exhaust gas that the catalyst temperature be raised as soon as possible since the start-up of the internal combustion engine. Namely, if the exhaust gas flows through the catalyst in a state of the catalyst temperature is not increased, there might be a possibility in which the unpurified exhaust gas is released intact into the atmospheric air.
Therefore, the catalyst known as a start catalyst may be provided in close proximity to an exhaust port of the internal combustion engine in order to purify the emission at the start of the internal combustion engine, whereby the temperature is increased upstream of an exhaust pipe at an early stage.
Even in the case of providing the start catalyst, however, it is invariably desirable that the catalyst temperature reaches faster the predetermined active region.
On the other hand, it is difficult in many cases in terms of designing a vehicle that the exhaust passageway extending from the internal combustion engine to the catalyst is formed rectilinearly without slightly bending the passageway. A flow of the exhaust gas might be deflected depending on a configuration of the exhaust passageway disposed upstream of the catalyst, resulting in an occurrence of such a phenomenon that a flow velocity of the exhaust gas flowing into the catalyst changes, a position in which the exhaust gas encounters the catalyst deviates, and so on.
Under this circumstance, for example, Japanese Utility Model Application Laying-Open Publication No. 1-119820 discloses a system in which a loss of pressure in each passageway through the catalyst support is reduced at the center of the catalyst support, and the flow of the exhaust gas is converged at the center, thereby uniformizing the flow velocity of the exhaust gas flowing into each passageway through the catalyst support even when a direction of velocity of the exhaust gas is toward an outer peripheral portion.
As a result of implementing a variety of examinations, the inventors of the present patent application, however, discovered that a heat spot is provided, a temperature thereof is raised intensively, and the heat evolved there is transferred to other portions, thereby attaining a much earlier rise in the temperature of the whole catalyst than by warming the whole catalyst with an uniformized flow velocity of the exhaust gas.